As a conventional method of measuring the inductance of a synchronous motor, for example, an inductance measuring method is disclosed in Patent Document 1, in which, by applying an ac voltage (so-called an alternating-current voltage) in a direction parallel to a dc axis rotating at an arbitrary frequency, an ac current having the same frequency component as that of the ac voltage is detected from a dc-axis current obtained by converting a motor current (a u-phase current and a w-phase current) detected by a current detector using a coordinate converter for converting it to a current on the dc axis as an application axis of the ac voltage, and then, the inductance is calculated from the maximum and the minimum values of the ac current and the ac voltage during a period when the dc axis rotates at an electrical angle of 180 degrees or more.
As a control device for a permanent-magnet-type synchronous motor, the following technology is disclosed in Patent Document 2. That is, by applying an ac voltage in a direction parallel to a permanent magnet of the synchronous motor, a d-axis current as a current component parallel to the permanent magnet of the synchronous motor is detected, thereby time-integrating the ac voltage to calculate a d-axis magnetic flux. According to the result, a hysteresis curve is drawn by plotting the d-axis current on the horizontal axis and the d-axis magnetic flux on the vertical axis, an average curve passing through the middle of the hysteresis curve is obtained, and, by adding an offset to the vertical axis of the average curve, the second average curve passing through the origin is obtained. Then, the slope of the straight line connecting a point on the second average curve corresponding to each d-axis current value and the origin is obtained as a d-axis inductance for each d-axis current value. Moreover, in addition to the d-axis inductance, by applying the ac voltage in a direction perpendicular to the permanent magnet of the motor, a q-axis current as the current component perpendicular to the permanent magnet of the motor is detected, thereby obtaining a q-axis inductance by a method similar to that of the d-axis inductance. That is, the inductance measuring method is disclosed therein.
In Patent Document 3, a method of identifying the inductance value of the motor is disclosed as follows. A high frequency rectangular wave signal is superimposed on a current command value or a voltage command value. When the high frequency rectangular wave signal is superimposed on the current command value, a rectangular wave signal component is extracted from a second current command value calculated so as to match a current detection value with a d-axis or q-axis current command value on which the rectangular wave signal is superimposed, a d-axis or q-axis inductance setting value is multiplied by a ratio of an absolute value of the extracted rectangular wave signal and an absolute value of the superimposed rectangular wave signal, whereby the inductance value of the motor is identified. On the other hand, when the high frequency rectangular wave signal is superimposed on the voltage command value, the rectangular wave signal component is extracted from the current detection value, and the absolute value of the extracted rectangular wave signal is divided by an absolute value of the rectangular wave signal superimposed on a d-axis or q-axis voltage command value, whereby the inductance value of the motor is identified.